Extrusion of peroxide crosslinkable polymer parts

Abstract

A process for the extrusion of peroxide crosslinked polymer parts, mainly tubes, in which the crosslinkable polymer is heated in an extruder in a controlled manner to a value above the crystallite melting point of the polymer, but below the crosslinking temperature, and fed continuously to an extrusion die to create the part and in which at least a partial crosslinking of the polymer is achieved in the extrusion die by heating of the polymer above a crosslinking temperature provision.

1. A method for extruding a peroxide crosslinked polymer tube, comprising:
 supplying a mixture to a screw extruder, the mixture comprising: a crosslinkable polymer, a crosslinking agent, and a stabilizing agent, wherein the polymer has a crystallite melting point and a crosslinking temperature, and wherein the screw extruder is a conically tapered screw extruder having at least two sections of differing spiral pitch; 
 heating the mixture in the screw extruder with an external heating unit to a temperature above the crystallite melting point but below the crosslinking temperature; 
 controlling the temperature of the mixture in the screw extruder with the external heating unit and an internal cooling unit; 
 continuously feeding the mixture from the screw extruder to an extrusion die, wherein a melting pressure before entry to the extrusion die is approximately 700-1500 bar; 
 heating the mixture in the extrusion die above the crosslinking temperature to effect at least a partial crosslinking of the polymer in the extrusion die, wherein the temperature (° C.) of the mixture in the extrusion die is at least 15% above the crosslinking temperature (° C.); and 
 discharging the mixture from the extrusion die, wherein the degree of crosslinking of the polymer on discharge from the extrusion die is above 60%. 
 
   
   
     2. The method of  claim 1 , wherein the crosslinking agent comprises organic peroxide. 
   
   
     3. The method of  claim 1 , wherein the melting pressure before entry to the extrusion die is approximately 1200 bar. 
   
   
     4. The method of  claim 1 , wherein said extruder is a twin screw extruder. 
   
   
     5. The method of  claim 1 , wherein the temperature of the polymer in the extrusion die is achieved by heating the extrusion die externally. 
   
   
     6. The method of  claim 1 , wherein the temperature of the polymer in the extrusion die is achieved by the induction of heat from the interior of the extrusion die. 
   
   
     7. The method of  claim 1 , wherein the temperature (° C.) of the polymer in the extrusion die is not higher than 60% above the crosslinking temperature (° C.) of the polymer. 
   
   
     8. The method of  claim 1 , wherein the temperature (° C.) of the polymer before entering the extrusion die is not higher than 30% above the crystallite melting point (° C.) of the polymer. 
   
   
     9. The method of  claim 1 , wherein the crosslinking temperature (° C.) of the polymer is approximately 30% above the crystallite melting point (° C.) of the polymer. 
   
   
     10. The method of  claim 1 , wherein the crystallite melting point of the polymer is approximately 125-140° C. 
   
   
     11. The method of  claim 1 , wherein the crosslinking temperature of the polymer is approximately 165-185° C. 
   
   
     12. The method of  claim 1 , wherein the tube is maintained at a temperature above the crosslinking temperature after discharge from the extrusion die. 
   
   
     13. The method of  claim 1 , wherein the tube is cooled after crosslinking.